Related Art
A film of diamond-and-the-like is superior in hardness, wear resistance, electrical insulating properties or thermal conductivity and hence is currently employed in, e.g., cutting tools, optical materials or electronic materials. In order for a member coated with such film of diamond-and-the-like to be used as a cutting tool or the like for a prolonged period of time, it is mandatory that the substrate surface be coated with diamond excellent in tight adhesion characteristics.
For this reason, it has been proposed in, for example, JP Patent KOKOKU Publication No. 60-59086 and JP Patent KOKAI Publication No. 63-306805, to employ a substrate consisting of a sintered body of ceramics, such as silicon nitride, a hard material which has a thermal expansion coefficient approximate to that of diamond and on which the film of diamond-and-the-like can be directly applied easily.
Silicon nitride has a thermal expansion coefficient approximate to that of diamond and is used as a sintered substrate with the least risk of peeling off of a diamond film deposited thereon under thermal stress which occurs subsequent to diamond coating. However, since silicon nitride can be sintered only with difficulty, sintering aids are added thereto. These sintering aids remain as a glassy phase in the grain boundary after sintering of silicon nitride ceramics.